First try restarting your phone if you haven't already. Also, in iOS 11 you can delete system apps you don't use so try removing apps you may not need. Otherwise you may have to wait for the next iOS update.

I have iPhone 6 plus with 16g as well, you are not suppose to be updating to the latest IOS with it because the latest system takes up 13GB or 11+GB on older system. You can only store few photos and prabably 1-2 apps. I purchased another phone because of this.

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